1. Field of the Invention
This invention relates to a modified ultrahigh-molecular-weight polyolefin having excellent stretch-moldability, and to a process for producing a stretch-molded article of the modified polyolefin.
2. Description of the Prior Art
It has already been known to produce molecularly oriented molded articles having high modulus and tensile strength by molding ultrahigh-molecular-weight polyolefins into fibers, tapes and other articles and stretching them. For example, Japanese Laid-Open Patent Publication No. 15408/1961 discloses that a dilute solution of an ultrahigh-molecular-weight polyolefin is spun and the resulting filaments are stretched.
However, a stretched article of such ultrahigh-molecular-weight polyethylene like that of general-purpose polyethylene is non-polar, and has poor adhesion to another substance. Thus, even when it is used as a reinforcing material, its properties are unlikely to be fully utilized.
On the other hand, for improvement of the adhesion of a nonpolar polymer such as polyethylene, modification of polyethylene by mixing it with an unsaturated carboxylic acid derivative such as maleic anhydride under heat (Japanese Patent Publication No. 6384/1964) is a best known method. Even if this method is applied to ultrahigh-molecular-weight polyethylene, it is difficult to graft-modify in an extruder or the like because it has an extremely high molecular weight unlike general-purpose polyethylene and has a high melt-viscosity. Furthermore, since polyethylene undergoes a crosslinking reaction at the time of graft modification, modified ultrahigh-molecular-weight polyethylene that might be obtained by such a method will further increase in molecular weight, and partly undergo gellation. Consequently, it is hardly possible to spin and stretch such a modified ultrahigh-molecular-weight polyethylene.
In an attempt to eliminate these defects, the present applicants previously proposed a method which comprises melt-kneading ultrahigh-molecular-weight polyethylene and an unsaturated carboxylic acid as one monomer having an olefinic unsaturated bond with a specific aliphatic hydrocarbon compound (Japanese Laid-Open Patent Publication No. 57604/1986), and a method which comprises kneading ultrahigh-molecular-weight polyethylene, a silane compound, a radical initiator and a diluent (Japanese Patent Application No. 268356/1985). These methods can prevent gellation in an extruding stage. However, when ultrahigh-molecular-weight polyethylene is mixed with an ethylenically unsaturated monomer and then melt-kneaded, the uniformity of an extrudate obtained from the kneaded mixture is unsatisfactory and this causes various defects such as reduced operability in spinning, and filament breakage in the subsequent stretching operation.